I've actually seen this happen on occassion as well. In my experience, those two reports are the most inconsistent in the system. However, the amounts are usually not that far off. Unfortunately, there there isn't one thing that always causes it, making a quick fix kind of tough. For example, in earlier versions of 5.3, there used to be an issue with the smart tax. If food was placed on a bill with Alcohol that was normally tax inclusive, tax should be paid by the guest. But in somes case, that check would later end up being split, and the food was moved to another check, leaving only the alchohol. Which technically means tax should be paid by the establishment. In instances where this exact scenario happened, the reporting would become inaccurate.. one report would say the tax should be paid by the guest, and the other report would say the tax was inclusive. However, I doubt this is your issue, since it would be unlikely to amount to that great of discrepancy.
It's been my experience that most of these problems can be traced to settings on the back of house, with some exceptions. I know this isn't much of answer, but I would not worry about it if it has only happened this one time. When I troubleshoot these annoying reporting issues, I like to have at least two instances where the reports were off. That way, you can audit the two server's activities on those days (yeah, fun!), and see what was similar about them. Then, you can generally find out where the reporting problem is located, and at the very least have a starting point.
Right now, with only one instance to go on, it's like looking for a needle in the haystack.
Just out of curiosity, which of these two reports was the one you believe to be correct? The front of house, or the back of house?